The invention relates to a gear shift device for a vehicle.
A gear shift device of this type is known from U.S. Pat. No. 4,593,580. If the driving gear wheel is engaged with a first driven gear wheel and the driving gear wheel requires to be disengaged from this driven gear wheel and the driving gear wheel requires to be engaged with a second driven gear wheel, the torque in the drive shaft is first reduced to zero, whereupon the driving gear wheel is brought out of engagement with the first driven gear wheel. The drive shaft""s frequency of rotation is then matched to the frequency of rotation of a second driven gear wheel by means of a regulating device in such a manner that their circumferential speeds are equal, whereupon the driving gear wheel is brought into engagement with it.
Since the frequency of rotation of the driving gear wheel or the second driven gear wheel, or both of these gear wheels vary during the gear change in order to achieve such a match, either due to the fact that the engine""s power or the vehicle""s speed and thereby the frequency of rotation of the driven gear wheels are altered relatively quickly, the interval in which a gear shift is possible may be very small. Since the vehicle""s driver constantly exerts a force on the gear lever in order to achieve the gear shift, i.e. attempts are made to bring the gear wheels into engagement, a continuous sliding or scraping of the gear wheels"" teeth against one another can easily occur before the frequencies of rotation of the gear wheels have become matched to such an extent that a gear change can take place. Furthermore, a fracture of teeth may occur if the gear wheels are engaged before their frequencies of rotation have been completely matched.
In manually operated gearboxes the displacement speed for the gear wheel which is moved by the slave cylinder during the engagement is a function of the force which is exerted on the gear lever. Thus the gear shift operation may not be executed during the above-mentioned interval and the gear wheels may slip or scrape against one another.
For example, if this force is great the gear shift may be performed too quickly. If, on the other hand, the force is small, the result may be that the gear shift operation is not started or completed within the above-mentioned interval.
A hydraulic gear shift device with double-acting hydraulic cylinders is known from the applicant""s Norwegian patent NO 171426.
An object of the invention is to provide a gear shift device of the type mentioned in the introduction, which to a far less extent is encumbered by the above-mentioned disadvantages.
In accordance with the foregoing object, this invention provides a gear shift device for a vehicle having a manually operated gearbox, an ingoing drive shaft in the gearbox which is connected with an engine of the vehicle, an outgoing driven shaft from the gearbox, a hydraulic gear shift device which is connected to a selector device, the gearbox having at least one driving gear wheel which is connected to the drive shaft and at least a first and a second, driven gear wheel which are connected to the outgoing driven shaft and by means of the gear selector device may optionally be connected to the drive shaft, a computer which is connected to (i) a gear shift operation-sensor device which is arranged to establish the occurrence of a gear shift operation initiated by means of the selector device and a gear into which a shift is to be made, (ii) a power control device for controlling the engine""s power, (iii) a torque establishing device for establishing the value of a torque which is exerted on the drive shaft, and (iv) frequency of rotation sensors for establishing the value of the frequency of rotation of the drive shaft and the driven gear wheels or the driven shaft respectively, the computer being arranged to receive impulses from the gear shift operation-sensor device, the torque establishing device and the frequency of rotation sensors and to transmit control impulses to the power control device, wherein said hydraulic gear shift device includes a master cylinder which is connected to said selector of the hydraulic gear shift device, a slave cylinder which is connected to a selector device of the gearbox, and at least one hydraulic line that connects the master and slave cylinders with each other, and wherein the hydraulic gear shift device comprises a shut-off valve which is mounted in the hydraulic line, and the computer is arranged to transmit impulses for controlling the shut-off valve based on the received impulses.